The background of this invention recognizes that AA content in polyester resins is an important factor when the end-use of the resin is concerned. Generally, resin manufacturers wish to produce a resin with as low an AA content as possible within the economics of a continuous commercial process.
In order to manufacture a polyester resin which can be transformed into a preform, the manufacturer is required to solid state polymerize (SSP) the resin to a sufficiently high viscosity index (IV). In order to conduct the solid state process, a pre-polymer or melt phase polymer must first be prepared using certain monomeric materials. The first reaction is sometimes referred to as melt phase polymerization. After such polymer is present in the melt condition, it is extruded, pelletized, and then cooled in a liquid, such as water. These monomeric materials are exemplified in the art by ethylene glycol, terephthalic acid or its ester, and other co-polymeric materials such as other dicarboxylic acids, multi-carboxylic acids, or other diols, triols, or multi-hydroxyl monomers.
However, traditional melt-phase polymerization entrains undesirable products and unreacted monomer components. AA, a noxious by-product of the melt polymerization process, can spoil the flavor and aroma of contained beverages, especially mineral water. Producers of containers for drinking water and soft-drink containers desire an AA content of less that 1 .mu.g/l, parts per billion (PPB), evolved into the headspace of an empty container. To achieve such a low AA content container, the solid state polymerized resin AA content should not exceed 1 part per million (PPM).
Multi-layer containers can also be used to avoid the problem of by-product AA contamination of drinking water and beverages. Each layer is composed of polymers with specific properties. Such containers are more expensive to make and raise concern with recycle of post consumer containers.
In. U.S. Pat. No. 4,340,721, a polyester is disclosed having a low acetaldehyde content. The intrinsic viscosity of the melt condensate is limited to 0.75 to 0.90 of the maximum possible IV, i.e. of about 0.6 dl/g. It has now been discovered that if the melt condensate is constrained to even a lower IV vis-a-vis the end point IV, the longer time necessary to SSP the polymer results in lower resin incipient AA content. This reduces a significant contributing factor of AA content in a resultant bottle made from a preform. The resultant bottles contain a maximum AA content of 1 .mu.g/l (PPB).
Finally, in U.S. Pat. No. 4,154,920 a pre-polymer is maintained at 0.1 to 0.3 dl/g prior to SSP. When one starts at this level of IV as the feed material to the SSP, the resin becomes difficult to pelletize.